1. Field of the Invention
The present invention relates to a method of controlling the power of a fixing device of an electro-photographic image forming apparatus, and more particularly, to a method of controlling the power of a fixing device employing a heat pipe in a print standby mode, by which instantaneous heating and low power consumption are possible.
2. Description of the Related Art
Electro-photographic image forming apparatuses include a fixing device to fix a toner image on a sheet by temporarily fusing toner image powder on the sheet on which the toner image has been transferred. The fixing device includes a fusing roller to fix the toner on the sheet and a pressure roller to press the sheet against the fusing roller.
FIG. 1 is a schematic longitudinal cross-section of a conventional fusing roller in which a halogen lamp serves as a heat source. FIG. 2 is a schematic latitudinal cross-section of a fixing device adopting the fusing roller of FIG. 1.
Referring to FIG. 1, a fusing roller 10 includes a cylindrical roller 11 and a halogen lamp 12 installed at the center of the inside of the cylindrical roller 11. A coated layer 11a (for example, coated with TEFLON) is formed on the circumference of the cylindrical roller 11. The halogen lamp 12 generates heat within the cylindrical roller 11, and accordingly the cylindrical roller 11 is heated by radiant heat from the halogen lamp 12.
Referring to FIG. 2, a pressure roller 13 is located beneath the fusing roller 10 while having a sheet 14 between the pressure roller 13 and the fusing roller 10. The pressure roller 13 is elastically supported by a spring 13a and presses the sheet 14 between the fusing roller 10 and the pressure roller 13 against the fusing roller 10 with a predetermined pressure. Toner image powder 14a formed on the sheet 14 is fixed on the sheet 14 by the predetermined pressure and heat while passing between the fusing roller 10 and the pressure roller 13.
At one side of the fusing roller 10 are installed a thermistor 15 to measure a surface temperature of the fusing roller 10, and a thermostat 16 to block power supplied to the halogen lamp 12 if the surface temperature of the fusing roller 10 exceeds a predetermined temperature. The thermistor 15 measures the surface temperature of the fusing roller 10 and transmits an electrical signal of the measured surface temperature to a controller (not shown) of a printer (not shown). The controller controls the amount of power supplied to the halogen lamp 12 depending on the measured surface temperature in order to maintain the surface temperature of the fusing roller 10 within a predetermined temperature range. If the temperature of the fusing roller 10 exceeds a temperature limit due to malfunctions of the thermistor 15 and the controller, the thermostat 16 blocks power flowing to the halogen lamp 12 by opening a contact (not shown).
The fusing roller 10 employing the halogen lamp 12 as a heat source consumes a great amount of power and has a low temperature increase rate. In particular, when the power of an image forming apparatus is turned on, a significantly long warm up is needed.
FIG. 3 is a graph showing a temperature profile with respect to a timing of alternating current (AC) power supplied to the halogen lamp 12 of the conventional fusing roller 10. Referring to FIG. 3, t1 denotes a time to operate an image forming apparatus, and t2 denotes a time when the temperature of the fusing roller 10 of the image forming apparatus reaches a target fusing temperature (185° C.). The interval between t1 and t2 denotes the warm-up period of the fusing roller 10. During the interval between t1 and t2, an AC voltage is continuously supplied to the halogen lamp 12, a temperature increase rate of the fusing roller 10 is 4-5° C./sec, and the warm-up period is about 35-40 seconds.
The interval between t2 and t3 denotes a print mode interval, and t3 denotes a time when the image forming apparatus enters into a print standby mode. During the interval between t2 and t3, the supply of power (i.e., the turning on and off of power) to the halogen lamp 12 is controlled in order to maintain the temperature of the fusing roller 10 at a predetermined temperature. For ease of illustration, frequent AC power on-off control is indicated by AC power ‘on’ in FIG. 3.
At t3, power is not supplied to the halogen lamp 12, and thus the temperature of the fusing roller 10 decreases. Given that the temperature rising rate of the conventional fusing roller 10 is 4-5° C./sec, and the time to change from the print standby mode to the print mode is less than 10 seconds, the temperature of the fusing roller 10 must be maintained at about 140° C. Hence, if the temperature of the fusing roller 10 is cooled from 185° C. to about 140° C., power is supplied to the halogen lamp 12, at t4. The supply of power (i.e., whether to supply power or not) to the halogen lamp 12 is controlled to maintain the temperature of the fusing roller 10 at the predetermined temperature. For ease of illustration, frequent AC power on-off control is indicated by AC power ‘on’ in FIG. 3.
In the case when the halogen lamp 12 is used as the heat source of the fusing roller 10, since a temperature of the fusing roller 10 rises at 4-5° C./sec, the surface temperature of the fusing roller 10 must be maintained at a relatively high temperature, about 140° C., in order to adjust a first print out time (FPOT) during a change from the print standby mode to the print mode in 10 seconds. Thus, a great amount of power is consumed in the print standby mode. Generally, in the case of a halogen lamp of 400 to 500 Watt/hour, power of about 250 Watt/hour is consumed in the print mode, and power of about 70 Watt/hour is consumed in the print standby mode.